Ground markers are a ubiquitous article in everyday life. Little noticed and taken for granted, ground markers can be found everywhere, on public streets, freeways, shopping center parking lots and airport runways, to name several such places. The number of ground markers in use and their cumulative cost is staggering to imagine.
The art of ground marker production has been the subject of a number of patents, among them U.S. Pat. Nos. 2,699,982; Re. 16,131; 1,708,369; 1,878,691; 5,104,256; 1,661,242; 3,392,639; 3,901,583; and 5,354,143. The improvements to ground markers are often addressed to three different features: visibility, durability and cost of manufacture.
The purpose of a ground marker is to mark or delineate traffic lanes or otherwise control and influence traffic movement. To serve this purpose, a ground marker must be highly visible. Visibility can be enhanced by the use of a reflective coating on the marker, or by the use of retro-reflectors, a device that reflects light back to its source. U.S. Pat. No. 5,104,256 teaches a low profile pavement marker that has a smooth upper reflective surface which scatters overhead light in all directions for daytime visibility, and a retro-reflective strip (to reflect light in the direction of its source) for night-time visibility.
A ground marker must also be durable to endure the natural elements and the constant wear and tear wrought by the friction of vehicle wheels. U.S. Pat. No. 3,392,639 teaches utilizing a shell of synthetic resin that is filled with material to form a solid core. The core reinforces the shell to provide a solid structure which initially is capable of withstanding forces applied to the outer surface of the marker. However, the resin itself is subject to wear and tear and the constant forces and vibrations exerted on the surface of the marker may eventually cause the inner resin core to crack or shatter, thereby weakening the entire structure. U.S. Pat. No. 5,104,256 suggests the use of ceramic in the manufacture of ground markers because of its long life and maintenance of reflective qualities and because the ceramic construction eliminates rust and corrosion. However ceramic is expensive compared to other materials such as plastic, metal, metallic/plastic compositions and alloys.
Durability can also be enhanced by the use of supports such as those used in U.S. Pat. No. 5,354,143. The ground marker of this patent includes a base block with radially aligned ribs. A cap structure is applied to the base block in situ. The ribs are supposed to aid in preventing cracking of the base block.
Given the great number of ground markers that are in use, the cost of manufacture becomes a major factor. A savings as small as a cent per unit ground marker can result in substantial savings for both the manufacturer and the purchaser when the large quantities of ground markers are taken into consideration. As indicated earlier, ceramic material offers durability, ease of maintenance, reflectivity and resistance to rust and corrosion. Plastic is cheaper but is less durable and is prone to breakage. Metal is subject to rust and corrosion and is malleable under certain conditions.